Once again missing the chance for counterplay with …a5-a4.
19. f2f3 White's pawns have lost some of their mobility, but the rest of the advantage is maintained: more space and the possibility of attacking on d6. White should still be alert to always have the option of responding to a5-a4 with b3-b4. 19... c8c7  20. c2d2  20... d7e6 Without caring about the d6-pawn. 21. c3e2 